Prior art teaches the practice of ellipsometric measurements with a relatively low spatial resolution of approximately 30 microns or larger. The spatial resolution of 30 microns or larger is generally a direct consequence of two ellipsometric measurement requirements. The two ellipsometric measurement requirements are (1) a relatively large angle of incidence of a beam on the surface of the object and (2) the angle of incidence is relatively well confined, e.g., ≲0.1 radians.
The relatively low spatial resolution property of the prior art sets the limits on the spatial resolution that is available to examine non-uniform surface properties of the substrate.